This invention relates to electrophoresis in general and, in particular, to an electrophoretic system employing a capillary with a conductive tip.
Capillary electrophoresis (CE) in narrow channels has proven useful as an efficient method for the separation of samples. An electric field is applied in the channel containing an electrolyte and a sample. The electric field causes the sample to separate its components which can be detected and collected if desired. CE is advantageous because it requires only very small sample volumes, such as the contents of a cell or similar subcompartments.
In the electrophoresis process, a high voltage on the order of several tens of thousands of volts is applied between the inlet and the outlet of a capillary channel in order to cause the sample to migrate and separate in a reasonable time and with reasonable resolution. In some conventional electrophoretic schemes, a capillary tube is employed having an inlet end and an outlet end. The tube is filled with an electrolyte such as a liquid buffer or a gel and a sample is introduced into the inlet end. Both the inlet and outlet ends are then immersed into buffer solutions and a high voltage is applied between the two buffer solutions in order to cause a high electric field in the capillary tube.
It is frequently desirable to make reliable electric contact between an electrolyte-filled capillary separation channel and a minimum amount of electrolyte contained in an inlet or outlet electrolyte buffer reservoir. A particular problem is encountered in electrokinetic sample injection where only a small volume of sample solution is available for injection. This is illustrated in reference to FIGS. 1A-1C below.
In electrokinetic injection, the inlet end 2a of a capillary 2 is immersed in a sample 14 and an electrical potential is applied across the electrolyte in the capillary so as to draw a portion of the sample 14 into the inlet end 2a. In order to apply an electrical potential between the two ends of the capillary, an electrode 18 is also submerged into the sample 14 in the configuration shown in FIG. 1A. The sample 14 is contained in the container 17. In actual operation, end 2a and electrode 18 are placed side by side and the container 17 may be moved upwards until both end 2a and the tip of electrode 18 are submerged in sample 14. Then an electrical potential is applied across the ends of the capillary using electrode 18 in order to inject a plug of sample into the inlet end 2a.
The above-described design and operation work very well for sample volumes greater than a few microliters but becomes less reliable as the sample volume is reduced. The problem which arises as the sample volume becomes increasingly small is depicted in FIGS. 1B, 1C. As illustrated in these two figures, any misalignment between the electrode 18 and the capillary 2 can cause the injection mechanism for the sample to fail. In FIG. 1B, for example, the electrode 18 does not extend as far downward as the capillary inlet end 2a. In this case, only the inlet 2a is submerged in the sample solution during the attempted injection process. The process in this case would fail since electrode 18 would fail to assist in applying an electrical potential across the ends of the capillary. In FIG. 1C, the capillary end 2a does not reach the sample solution 14 even though the tip of electrode 18 does reach the sample solution. Therefore, the sample injection would again fail.
In addition to sample injection, there may be applications where it is desirable to make reliable electrical contact between an electrolyte-filled separation channel and a small amount of electrolyte contained in an inlet or outlet electrolyte medium such as a reservoir. It is therefore desirable to provide an improved electrophoretic system in which the above-described difficulties are avoided.